Ibn Jebreen on sexual intercourse between a human being and a jinn [fire spirit]:

Question: Sexual intercourse between a human being and a jinn [fire spirit] does occur in reality, but rarely. The question is, does the occurrence [of sexual intercourse] between a jinn and a human being require ghusl [washing the body to purify it after a sexual act]?

Answer: It is known that jinn are spirits without need for a body. However, God decreed that they would speak audibly and appear in various forms and guises, just as he decreed that they would enter the bodies of humans in order to overcome the souls of humans. It is usually the case that male jinn enter only the bodies of female humans, and female jinns enter the bodies of male humans, where they find pleasure and sexual desire. It has been known for sexual intercourse to occur between humans and jinn when jinn appear in human form and speak to the people they desire, then draw up an Islamic marriage contract with female humans. They acquire these women by calling them, or with special hand signals, or with a rod, etc. Otherwise a jinn with the appearance of a woman comes to [a man], and he has sexual intercourse with her the way he would with his human wife, and he finds tangible pleasure in this and emits the known substance. I don't know if children are ever produced from these kinds of unions?

This is what trusted sources have related to us, but this is rare. Similarly, they [jinn] may kidnap a female human and keep her amongst themselves so that her family misses her and she marries one of them, who has sexual intercourse with her the way a man does with his woman. Some women have related that they are visited by jinn who speak to them or appear when they are alone and take them by force, like a husband. No one besides [these women] can see him, since he disappears before other members of the household. There is no doubt that when this known form of sexual intercourse occurs between a man and a jinn woman or a jinn man and a human woman, it results in major ritual impurity and requires ghusl, since either real penetration or ejaculation has occurred. And God knows best.